Unsaturated polyesters can be used as impregnating resins in the electrical industry, particularly for impregnating wound items such as electrical coils and windings of magnet wires in electrical devices, as well as for casting of electrical devices such as stators, rotors and transformers, in order to obtain mechanical toughening and fixing.
Impregnation materials based on unsaturated polyesters dissolved in styrene typically show excellent impregnation results due to their low viscosities and also good mechanical toughness. However, such systems show emission of styrene upon curing of the impregnation material, resulting in a significant amount of volatile organic compounds (VOC). Therefore, there is a strong demand for tough impregnation materials with low to zero emissions that offer excellent impregnation qualities.
Mechanical properties like toughness can e.g. be improved using fillers, however, impregnation materials containing such fillers are typically not very homogeneous, and their viscosities are typically high. This could cause problems regarding storage conditions of the composition and can additionally lead to failure of penetration into the cavities of electrical windings as well as to failure of impregnation quality during operation of the electrical device.
It is well-known in the art that one way to achieve a low viscosity system comprising unsaturated polyesters is the incorporation of non-polar moieties such as fatty acid derivatives into the unsaturated polyester.
Unsaturated polyester resins comprising polymeric fatty acids and/or other di-functional carboxylic acids are described in DE-A1695514, in order to improve the stability against hydrolysis. The resulting products are too soft for the use as impregnating resins in compositions for electrical insulation. DE-A1694951 discloses similar unsaturated polyesters comprising polymeric fatty alcohols instead of polymeric fatty acids.
Polycondensation of oligomeric or polymeric fatty acids and other multifunctional carboxylic acids like maleic acid or itaconic acid with multifunctional polyols is shown in EP-A0980882. After crosslinking these materials show good thermal and adhesive properties, however they are very soft and do not provide sufficient mechanical properties to be used as impregnating resins in compositions for electrical devices.
DE-A1770566 describes the preparation of unsaturated polyesters which can be used for impregnation of electrical devices containing an imide which is based on bi-cyclooctene tricarboxylic acid. The polyester may contain fatty acids. The crosslinked material shows good thermal, electrical and mechanical properties, however the resulting impregnating resin contains volatile reactive diluents resulting in a high volatile organic content (VOC) of the material.
DE-A2460768 describes unsaturated polyester imide resins for the use in impregnation compositions for electrical devices which may contain dimeric fatty acids. The resins are preferably dissolved in styrene resulting in a high volatile organic content (VOC).
WO2008/067967 describes unsaturated polyester polyols containing non-polar structures of dimeric fatty alcohols and dimeric fatty acids. These unsaturated polyester polyols are further converted into polyurethanes for coatings, elastomer and thermoplastic applications. For this purpose, the unsaturated polyester polyols must carry a minimum of two hydroxyl functionalities per polymer molecule. Such unsaturated polyester polyols are not suitable for the use in impregnating compositions for electrical devices, due to the high hydroxyl functionality which improves the interaction with electrical fields and thus decreases the insulation properties of the crosslinked material. Furthermore, the described unsaturated polyester polyols are too soft for the use in impregnating compositions for electrical devices.
It is known that low-viscous low-VOC impregnating resins typically contain a high level of reactive diluents resulting in poor mechanical properties such as strong brittleness.